1. Field of the Invention
The present invention relates to an improved igniter plug particularly useful to be employed for use in aircraft jet engines and gas turbine engines and burners.
2. Description of Prior Art
An igniter plug employed in a gas turbine engine of an aircraft is usually of the type of surface creeping discharge in which high voltage impressed between the center and ground electrodes results in spark discharge of increased energy. This causes erosion of the ground electrode upon extended use.
This situation arises in recent years because the igniter is continuously used or active to successively spark during flight for the fear of cessaton of combustion, in oppositon to the counterpart which is ignited only at the time of starting.
The ground electrode is continuously exposed to high energy spark during flight so that the wear of the electrode increases relatively quickly to shorten the service life due to erosion, even when the electrode is made of nickel-based alloy such as Inconel.
To improve this situation, an iridium insert ring is placed on the annular shoulder of the ground electrode as taught in U.S. Pat. No. 3691419 wherein the insert ring is welded to the shoulder only at the outer peripheral portion by means of electron beam welding to permit only a limited recrystalization.
It, however, is dificult for the igniter to provide good heat conduction between the insert ring and the shoulder due to the limited welding area therebetween.
Beam welding leads to a series of complicated operations, and it becomes impossible to weld when the insert ring is reduced to about 0.5 mm thickness.
Furthermore, a tubular insulator is required to abut against both the top end of the shoulder and the upper flat surface of the insert ring simultaneously at the time of assembly. This necessitates the use of an insert ring of dimensional accuracy, thereby increasing manufacturing cost.
Therefore, it is an object of this invention to provide an igniter plug which is capable of assuring good heat conduction between an insert ring and the stepped seat of a ground electrode so as to prevent undue wear due to erosion, and which is conducive to the long service life.
It is another object of the invention to provide an igniter plug which is suitable for mass production owing to the fact that electrical resistant welding is employable.
It is still another object of the invention to provide an igniter plug which is capable of reducing the thickness of the insert ring to save the amount of precious metal employed, thereby conducive to cost-saving.
According to the invention, an igniter plug comprising; a shell metal having a stepped seat at the inner side adjacent to the open end to form an annular ground electrode; a tubular insulator concentrically provided at the inner side of said shell metal to rest on said stepped seat through a oxidation and heat resistant insert ring; a center electrode disposed into said insulator to have one end distanced from said insert so as to form a surface discharge gap therebetween; and said insert ring being of platinium-based alloy, and welded to said stepped seat by means of electrical resistant welding so as to form a weld layer, at least extending between said ring and said stepped seat.